Method and apparatus for controlling the operating point of an amplifier stage

Abstract

At high frequencies, a video amplifier draws a higher current and normally is biased to provide such current. However, since high frequencies are rarely encountered, the present circuit controls the operating point as a function of the frequency. More specifically, a controlled variable which controls the operating point of the amplifier stage is obtained in dependence on the frequency to which the amplifier stage is driven.

I claim: 
     
       1. Apparatus comprising: an amplifier having an input and an output;   a source of an input video signal coupled to said input of said amplifier; said video signal having trace intervals in which picture information is present and retrace intervals in which video information is not present;   means for coupling said output of said amplifier to a load;   a source of a bias signal coupled to said input of said amplifier;   said amplifier including a first transistor having a first-input electrode, a second-output electrode and a third electrode, and arranged in a follower configuration; said first-input electrode being coupled to said input of said amplifier; said second-output electrode being coupled to said output of said amplifier; and said third electrode being coupled to a power supply terminal;   a sensing element connected between said third electrode of said first transistor and said power supply terminal; and   a feedback network, including a filter capacitor for developing a DC signal indicative of the current supplied by said amplifier to said load in response to said picture information represented by said video signal during said trace intervals, coupled between said sensing element and said source of said bias signal for modifying said bias signal as a function of said current.   
     
     
       2. The apparatus recited in claim 1 wherein: said amplifier also includes a second transistor of the complementary type with respect to said first transistor, said second transistor also having a first-input electrode, a second-output electrode and a third electrode, and also being arranged in a follower configuration; said first and second transistors being arranged in a push-pull configuration; said first-input electrodes of said first and second transistors being coupled together to said input of said amplifier; said second-output electrodes of said first and second transistors being coupled together to said output of said amplifier; and said third electrodes of said first and second transistors being coupled to respective power supply terminals;   a rectified version of said current supplied by said amplifier to said load being developed across said sensing element in response to said picture information represented by said video signal during said trace interval by virtue of the push-pull operation of said amplifier; and   said feedback network not containing any rectifying elements for developing said rectified version of said current supplied by said amplifier to said load.   
     
     
       3. The apparatus recited in claim 2 wherein: said first and second transistors are bipolar transistors having respective base, emitter and collector electrodes; said first-input electrodes are respective base electrodes; said second-output electrodes are respective emitter electrodes; and said third electrodes are respective collector electrodes.   
     
     
       4. The apparatus recited in claim 3 wherein: said load is a picture tube and said output of said amplifier is coupled to a cathode of said picture tube.